The present invention relates to heat resistant cast iron-nickel-chromium alloy, and more particularly to austenitic heat resistant cast iron-nickel-chromium alloy having the composition of Cr, Ni, and Nb which is excellent in creep fracture strength at high temperatures and in resistance to thermal impact or carburizing, with further use of the composition of N, Ti, Al and B, especially under the severe operating conditions at temperature above 1000.degree. C.
HK 40 which is a heat resistant cast iron-nickel-chromium alloy containing Ni and Cr (25Cr--20Ni steel, see ASTM A 608) and HP materials (see ASTM A 297) have been used as materials for ethylene cracking tubes in the petrochemical industries. With the elevation of operating temperatures in recent years, it has been required to improve the high-temperature characteristics of such materials. To meet this requirement, HP materials containing Nb have been developed and placed into use. However, with the recent tendency toward severer operating conditions, it is desired to provide materials which are superior to such HP materials containing Nb in respect of high-temperature creep fracture strength and resistance to thermal shock or carburizing.